Olefin polymerization catalyst systems may be divided into Ziegler-Natta and metallocene catalysts, and these highly active catalyst systems have been developed in accordance with their characteristics. Ziegler-Natta catalysts have been widely applied to commercial processes since they were developed in the 1950s. However, since a Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that a resulting polymer has a broad molecular weight distribution. Also, since a compositional distribution of comonomers is not uniform, there is a problem that it is difficult to obtain desired physical properties.
Meanwhile, metallocene catalysts include a main catalyst having a transition metal compound as a main component and an organometallic compound cocatalyst having aluminum as a main component. Such catalysts are single-site catalysts which are homogeneous complex catalysts, and offer a polymer having a narrow molecular weight distribution and a uniform compositional distribution of comonomers, due to the single site characteristics. The stereoregularity, copolymerization characteristics, molecular weight, crystallinity, etc. of the resulting polymer may be controlled by changing a ligand structure of the catalyst and polymerization conditions.
U.S. Pat. No. 5,032,562 discloses a method for preparing a polymerization catalyst by supporting two different transition metal catalysts on one support. This catalyst is prepared by supporting a titanium (Ti)-based Ziegler-Natta catalyst which produces a high molecular weight polymer and a zirconium (Zr)-based metallocene catalyst which produces a low molecular weight polymer on one support, and results in a bimodal molecular weight distribution. This catalyst is disadvantageous in that the supporting procedure is complicated and morphology of polymers is poor due to a cocatalyst.
U.S. Pat. No. 5,525,678 discloses a method for using a catalyst system for olefin polymerization, in which a metallocene compound and a non-metallocene compound are simultaneously supported on a support to realize simultaneous polymerization of a high molecular weight polymer and a low molecular weight polymer. However, there are disadvantages that the metallocene compound and the non-metallocene compound must be separately supported and the support must be pretreated with various compounds for supporting.
U.S. Pat. No. 5,914,289 discloses a method for controlling a molecular weight and a molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and a process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 2003-12308 discloses a method for controlling molecular weight distributions of polymers, in which the polymerization is performed while changing a combination of catalysts in a reactor by supporting a dinuclear metallocene catalyst and a mononuclear metallocene catalyst on a support together with an activating agent. However, this method has limitations in simultaneously realizing the characteristics of respective catalysts. In addition, there is a disadvantage that the metallocene catalysts are departed from a supported component of the resulting catalyst to cause fouling in the reactor.
Therefore, to solve the above drawbacks, there is an ongoing demand for a method for preparing polyolefins with desired physical properties by easily preparing a supported metallocene catalyst having excellent activity.
Among above attempts, however, very few catalysts have been practically applied in commercial factories, and thus preparation of catalysts showing more improved polymerization performance is still required.